General PA0 The Problem PA0 Solution to Problem
The superior lumen per watt characteristic of fluorescent lamps has for decades prompted research on ways to operate these lamps from a DC supply. These applications included the transportation industry (trains, transit cars, buses and airplanes) and the portable lighting industry. In these applications, no AC power is available and therefore the premium cost of these inverter ballasts was justified since the only alternate light source was the incandescent lamps (about 15 lumens per watt). When compared with fluorescent lamps of about 50 lumens per watt and about 10 times the life, the additional inverter ballast cost was justified.
It has been demonstrated as early as the early 50's, that the fluorescent lamp, when properly operated at frequencies above 15 KHZ, would demonstrate about 15% improvement in the output lumens per watt over 60 HZ operation. This well recognized fact, plus the present impetus on energy saving, has been the driving force behind multi-million dollar research and development efforts to apply high frequency lighting to commercial, industrial, and consumer applications. To date, there has been limited success in this effort. The reason for this record can be understood by studying the complexity of the problem added to the economics of the situation. Many efforts produced costs many times that of the 60 HZ Ballast counterpart with efficiencies, or ballast losses comparable or worse than the 60 HZ Ballast. Further, 60 HZ Ballast manufacturing has generally responded with better steel and more copper to improve their efficiency.
The problem of making High Frequency available for general fluorescent lighting can be defined in the following categories:
A. Efficiency - must approach 95%. This makes payback an economic reality. PA1 B. Cost - The cost of the High Frequency Ballasts must be no more than 2 or 3 times the cost of the 60 HZ Ballast. PA1 C. Reliability - The inverter ballast must match or better the 60 HZ Ballast. PA1 D. Life - Typical life must exceed ten years.
Many of the above problems are interdependent. For example, 95% efficient means extremely small losses and therefore low temperature rise, which generally means high reliability and long life. However, generally, cost tends to increase when the above objectives are addressed. We can then summarize our problem statement by saying the following: We must find a solution, if one exists, that will demonstrate the high efficiency and low loss with primary effort on production simplicity and low costs.
It has been generally accepted by these inventors since their first lighting of a fluorescent lamp with an inverter ballast, in the mid 50's, that resonance plays a dominant factor in ballast efficiency. However, maintaining resonance with component tolerances in production and during aging of the ballast appeared to be an impossible problem. In 1970, during work on the Coleman Camping Lantern ballast, the inventors were successful in providing resonant feedback which solved the above problem and produced efficiencies approaching 90%. This technology is used extensively in the transit industry and is the basis of Pat. No. 3,753,076. Following this work, a single transistor resonant feedback ballast was developed which approached 95% efficiency. This technology is the basis for most of the low voltage camping lantern ballasts made today, for example, Pat. No. 4,023,067. The resonant feedback promotes zero current switching of the transistor thereby providing the high efficiency.
The teaching contained herein goes an order of magnitude further in that, instead of promoting zero current switching, it assures it. Further, the start up transient is addressed in such a manner that the switching devices are less stressed during start up. Still, the above solutions have been accomplished in such a manner that a cost effective solution has been demonstrated. The 25% to 30% overall energy savings with no change in light output can easily justify the higher inverter ballast cost. Retrofitting field ballast should be possible with an approximate one year payback, depending upon local energy and labor costs.
The operation of the inverter in synchronism with the resonant current also automatically adjusts the switching frequency to the resonant frequency should the value of the inductor or capacitor degrade and therefor change. The switching losses will therefore be maintained at a minimum.